I did GA in Providence (near-ish to Boston) in 2001, and for Vertigo in 2005. I think you'll still have a great spot if you get there at noon. You won't be in the very front row, but I think you'll for sure be in the front half of people there, maybe even closer. The latest I ever arrived for a U2 GA show was around 1pm I think for the Pittsburgh 360 show, and even though my number was in the 600s, I was still in the third row center. That's the other thing to keep in mind - not everyone is going to want the same spot as you do. I'm always a little surprised to be reminded of that, because in my head everyone's all rushing to the same place, when in truth, a lot of people have different preferences. Some people like being near the b-stage more than the main stage, some people like the catwalk better than the stage, others like being more to the side where there's a little more breathing room than right in the middle. I'm always pleasantly surprised that it's never as crowded as I worry it's going to be.
When they were there for the Vertigo tour, the FleetCenter (now it's called something else, can't remmeber the new name offhand) was a little bit different than the other shows on the tour. The local security gave out numbered wristbands when people arrived, and then sent them away until 5pm. They started giving them out at 9am, so the small group of people who had gotten there before 9 got the first wristbands, and then people throughout the day got higher numbers, but I actually was able to go to most of my work day then, just had to be back by 5. (Anyone showing up late lost their place in line.) That was a local venue policy rather than U2's policy. It might be worth keeping an eye to see if that venue does that again.
They do still do that! And it's the TD Garden now.